Document how to use `harbormaster.sendmessage` to report lint and unit results
Summary: Fixes T7419. This doesn't really do anything, just adds documentation.
Test Plan:
- Read the documentation:
{F688899}
- Created a build plan which makes an HTTP request to `example.com` and waits for a result.
- Ran that build plan manually.
- Called `harbormaster.sendmessage` manually with the example lint/unit values to provide a result.
- Saw the results report correctly and the message ("fail") process as expected:
{F688902}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T7419
Differential Revision: https://secure.phabricator.com/D13789